Matching 67P's velocity and orbit has been tricky. Rosetta took a torturous, decade-long path that led it past earth three times and Mars once, as well as a couple of asteroids. Ahead of its final approach, it came at the comet from behind at about half-a-mile a second, before a series of braking maneuvers earlier this year.

"It was a fantastic achievement just to get this far, I think," says Matt Taylor, the European Space Agency's Rosetta's project scientist.

With its final thruster burn on Wednesday, it will bring itself into perfect step with the comet.